Exhaust systems are widely known and used with combustion engines. Typically, an exhaust system includes exhaust tubes that convey hot exhaust gases from the engine to other exhaust system components, such as mufflers, resonators, etc. Mufflers and resonators include acoustic chambers that cancel out sound waves carried by the exhaust gases. Although effective, these components are often relatively large in size and provide limited nose attenuation.
Attempts have been made to improve low frequency noise attenuation by either increasing muffler volume or increasing backpressure. Increasing muffler volume is disadvantageous from a cost, material, and packaging space perspective. Increasing backpressure can adversely affect engine power.
Another solution for reducing low frequency noise is to use a passive valve assembly. The passive valve assembly includes a flapper valve body or vane that is positioned within an exhaust pipe, with the vane being pivotable between an open position and a closed position. The passive valve is spring biased toward the closed position, and when exhaust gas pressure is sufficient to overcome this spring bias, the vane is pivoted toward the open position. When the exhaust gas pressure falls, the spring causes the vane to return to the closed position.
With the use of the spring, it is difficult to return the vane to a consistent closed position within the exhaust pipe. Further, while effective at attenuating low frequency noise, the introduction of the passive valve into the exhaust system presents additional noise challenges. For example, when the spring returns the vane to the closed position, closing noise is generated, which is undesirable.
Therefore, there is a need to provide a passive valve arrangement that can effectively and efficiently return a vane to a consistent closed position without generating additional noise. Further, the passive valve arrangement should minimize closing forces to improve durability of the passive valve.